Vacuum pump



. Jan. 29, 1963 F. J, ERHART ErAL 3,075,688

VACUUM PUMP Filed Nov. 2o, 1961 ,fv 45 i 54 l\'|"12 l ,a im T? y l f C? ff f l /6`l M l l El 3o` l M// 32v11 i III lli IJ l. l/

INVENTORS FRANCIS J. ERHART F75 MATTHEW W. DAVIS JR.

BY' mwew ATTORNEY York Filed Nov. Ztl, i961, Ser. No. 2153,543 2 Claims. (Cl. 2310-101) The present invention relates to vapor vacuum pumps and, more particularly to diffusion and ejector pumps of the type which utilize .a jet nozzle assembly comprising one or more jet nozzles.

Generally, the use of diffusion rand ejector pumps for creating vacuum involves the very disagreeable phenomenon of backstreaming which occurs when molecules of oil vapor migrate back into the vessel to be evacuated. Various measures have been taken to counter this movement of oil vapor, most notably, the use of highly cooled bafes and the like which are positioned between the vacuum pump and the vessel to be evacuated.

The disadvantages in using battles is that the conductance therethrough is seriously limited and this in turn greatly reduces the eective pumping speed of the pump and baille combination. Generally, the amount of backstrearning through a baille of conventional design is inversely proportional to the impedance of the baille, that is, the more the baffle is capable of reducing the backstreaming the less condu-ctance there is through the baille.

ther attempts to minimize back-streaming have involved the use of a high conductance bale and so-called cold caps on each of the nozzles from which the vapor jet issues, more notably, the top jet nozzle. These cold caps are cooled by various devices such as Peltier couples or cooling coils coiled around the cap and connected to an outside source of cooling medium. The cooling of these caps requires equipment that is relatively expensive and the conduits connecting the cooling devices to equipment located outside the pump extends through the wall of the pump housing thus offering an increased number of places where leaks may develop.

It is the principal object of the present invention to minimize back-streaming in a vapor type vacuum pump utilizing :a minimum of parts and at relatively low cost.

Other objects and advantages will become apparent after studying the following description and claims taken in conjunction with lthe accompanying drawings wherein:

FGURE 1 is vertical section through a diffusion vacuum pump showing the .association of the present invention with the pump; and

HGURE 2 is a fragmentary view, partly in section, of the top jet nozzle of the pump of FlGURE l, showing the present invention applied thereto.

Referring more particularly to FIGURE 1 of the drawing, there is shown a diffusion vacuum pump generally indicated by the reference numeral which includes a hollow cylindrical casing l2 with an integral bottom wall 14. The upper end of the pump is provided with a suitable inlet tiange 16 to which the cooperable iiange 1S of a vessel 29 to be evacuated may be secured. Arranged between the ilange i6 is ya bathe 22 which serves to condense the oi-l vapor which tends to mignate from the pump toward the vessel 2Q. The discharge side of the pump is provided with a pipe 24, one end o-f which opens into the lower portion of the casing 12.

Within the pump casing there is centrally arranged a hollow nozzle assembly comprising a plurality, and in this case four, of jet nozzles indicated at 26, 28, 3i), 32. Vapors rising in the interior of the nozzle assembly due to vaporization of pump iiuid are directed out of the respective oriiices in the downwardly directed jet nozzles. A suitl ?atented dan. 29, i963 able heater (not shown) located within the nozzle assembly and arranged on the bottom wall 14 serves to heat pump fluid to cause vaporization thereof.

in FlGURE 2, the jet nozzle 26 is illustrated as comprising a lip portion 36 and a frusto-conical skirt 3S spaced slightly radially therefrom. The illustrated jet nozzle arrangement is conventional and, as known in the art, hot oil vapors are driven upwardly through the chimney 4t)I of the nozzle assembly and expelled at great speed out of the annular spacing 42 between the lip 36 and the skirt 38. Normally the ygreater portion of these vapors lare directed downwardly through the pump houtsing in order to achieve pumping action, however, a small amount of the vapors turn the lower edge of the skirt and migrate upwardly toward the baille 22. This migratory oil vapor is 4condensed by the baiiie land oil droplets, resulting therefrom, will drop down into the heater portion of the pump to be vapo-rized.

In order to minimize backstreaming or the mignation of the oil vapor in the direction of the intake end of the pump housing, the top jet nozzle 26 is provided with a shield or cap 44 which conforms generally to the frustoconical shape of the skirt 38 but overlaps the lower edge thereof for a considerable distance by virtue of a vertical depending cylindrical projection 45. The lower edge of the portion 4S extends toward the fringes of the jet stream emanating strom the jet nozzle and serves to deileet the vapors which turn outwards around the lower edge of the skirt 38 and would normally drift upwardly toward the flange 16. rlhese migratory vapors which impinge upon the cap 4d and project-ion 45 heat the same and, in order to prevent rte-evaporation of the oils which may condense on the cap, the same is maintained in a condition where re-evaponation cannot occur.

in the present invention this is accomplished by preparing the cap 44 and the projection 45 as an insulated body and to this end, the under-surface 4S of the cap and the projection is made reflective and a pair of heat shields 50, 52 are provided in spaced relation between the cap and the ski-rt of the nozzle 25. Both the upper and under-surfaces of the shields S0, 52 are ma-de reilective. The reflective under-surface 48 of the cap and projection reflects heat therefrom :and tends to minimize the heat that would develop on the cap and projection by hot vapors emanating'from the jet nozzle 26.

This reduction in the amount of heat reaching the cap 4diand projection 45 permits the natural loss of heat through radi-ation from the jet nozzle 26 to the walls of the pump casing to maintain the temperature of the cap at a sufiiciently low point to prevent re-evaporation of the condensed vapor.

These surfaces also reiiect heat back into the jet nozzle ycausing it to operate at a somewhat higher temperature, thereby making the stream drier or free of liquid pump fluid, thus improving the quality of the jet stream expel-led from the nozzle throat. In eilect then the cap 44 and the projection 4S serves to deflect oil vapor which would deviate from the main jet stream of the pump and, to prevent the cap and projection from becoming another or secondary source of oil vapor, the cap 44 and the projection 45 is efrectively maintained at a temperature where re-evaporation `does not occur by the provision of the lower reflective surfaces of the cap and the shields 50, 52. With this arrangement, the need for extraneous cooling devices such as cooling coils, conduction cooling members, Peltier couples and the like for large scale `cooling is eliminated.

, Detlection of the oil vapors by the projection 45 also tends to reshape the otherwise useless fringe por-tion of the jet stream by directing these vapors back into the jet 'stream where they can perform useful pumping action.

In this manner, the speed of the pump will be increased somewhat over and above a nozzle arrangement without the cap.

The reflective characteristics of the lower-surface of cap 44 in conjunction with the lower reflective surfaces' of the shieldsSi-and 52 aid in maintaining the heat Within the jet nozzle 26. These characteristicsl of the surfaces also reduce the amount of heat reaching the cap 44. The shield 50 is mounted adjacent to and in spaced relation from the cap 44 and terminates along its lower end into a vertically depending cylindrical projection 54.

The second intermediate Vreflector shield 52 is mounted between the skirt 38 and the shield 50 and terminates in a vertical depending'projection 56'the lower edge ofwhich overlaps the lower edge of thefskirt' and is overlapped in turn by the lower edge of theprojection 54. Both-surfaces of the shield 52 and the-projection V54 are Vmade reilective with the under-'surface reflecting some'of the heat that may` radiate from the skirt 38'v and the upper surface maintaining coolness of the shield 52 by--functioning--as a poor emitter of heat. The reflective characteristics of the lower surface of the cap 44 andboth undersurfaces of the shields y50, 52 and their respective projections 54, 56 arersuch that -the maximum amount of heat possible is retained within the jet-nozzle 26v and kept away from the c'ap 44. As shown in FIG. 2., the lower edgesof the 'projections Y54 Aand 56 ter-minate'at points considerably higher than'the lower edge of theA projection 74S thus allowing a considerable 'areaof the projection 45 to be exposed to the fringe vportion of the jet stream. In addition, with Vthe edges of the projections 54, V56 located above the plane of the inner surface of the skirt 38, the

projections 54,- 516 receive a very slight amount of vapor trated and comprise vertically projecting screws, S8 (only one shown) secured to and extending from the skirt 38.

'The screws 58 project through suitable apertures formed 'in the shield 52 and retain the latter by'a nut located on either side of the shield S2. Similarly, screws 60 retain the shield 50 upon the shield 52 and screws 52 support the cap 44 upon the shield 50. Insulating washers may be applied to the screws A58, 60 and 62 between their respective nuts in order to minimize heat conduction' between the shields, the cap and the skirt.

The present invention is distinguished from the invention ndisclosed and claimed in the copending patent application Serial No. 153,544, led November 20, 1961, assigned to the same assignee in that the present invention requires an insulated cap formed with a generally vertically extending cylindrical projection and at least two heat reflective shields. Since the projection is bombarded withvapor molecules at a more direct angle, the amount of heat produced on the projection by the condensed vapor is relatively large or at least largerthan if the lower edge of the cap was: formed with -a straight conical shape ofthe type disclosed in the-above referred to copending patent application. At least two heat reflective shields are necessary in the present invention to lessen sufficiently the trans- Yfer of heat to the cap from-the jet nozzle in order to compensate for the additional heat transfer from the condensed vapor. With a straight conical shape cap, as disclosed in the copending application, the vapor molecules strike the inner surface of the cap at a more glancing angle resulting in a lesser amount of vapor beingy condensed and a correspondingly lesser amount of heat being transferred to the cap. With this arrangement, a single heat reflective shield will suihce to lessen the transfer of heat to the cap. On the other hand, in order to project into the fringe portion of the jet stream, the straight conical cap must be extended further than the cylindrical projection of the present invention resulting in a relatively large base diameter for the cap of straight conical design. This will lessen the pumping speed of a diffusion pump as compared to the speed of the pump lwhen using the cap of the present invention. Nevertheless, use of a larger diameter cap of the straight conical design does eliminate more back- :streaming than is the case with the smaller diameter cylindricalprojection design of the present invention.

4-In order'to insure furthermaintaining-the cap'44 in a condition to prevent re-evaporation of condensed Vapor, the'upper surface'ofthe cap and projection 45 y.may be fcoated with a material having'ahgher emissivity than. the

normal metallicsurface of thecap. An example of such a high'ernissivitymaterialfor use V/withla cap made from aluminum would be a. black dyedanodized. surface.

From .the foregoing it 'willi be apparent that. thepresent `iriventionmay'be`applied to any orall. of the jetnozzles of the nozzle assemblyof a diffusionpump. .The provision of aniinsulated 'cap for' deectin'goil vapor that wouldother `wise migrate upstream 'of the pump and will`permit the'use of a baffle l22V having a greaterconducta'nce. Maintainingthe insulated cap in'a'condition such as to prevent re-evaporation of oil thereon is accomplished by providing a pair of reflective shields mounted between the jet nozzle and the'cap and by having the surfaces of the cap and -the shields which face the jet nozzle made reective. lIn

the use of vthe present invention, there is no need to employextraneous cooling devices to effect cooling of the cap 44.

We claim:

l. In a vapor vacuum pump, a pump casing having an "inlet, an outlet, and a boiler'for heating pump duid, a

nozzle assembly mounted in the pump casing for providing a vapor jet stream, said nozzle assembly having at least one jet nozzle comprising a lip portion and a skirt between which the jet of vapor is expelled for pumping action, a cap surrounding said jet nozzle in spaced relation to said skirt and being formed with'a generally vertical depending edge portion extending below saidV skirt and into the fringe portions of the jet stream, said cap having the surface which is adjacent the jet nozzle reflective thereby to reflect some of the heat emanating from thetjet nozzle which would thermally affect said'cap, and at least two reflective shields mounted in spaced relation to each other vertical depending edge portion extending below said skirt Vand into the fringe portion of the jet stream, said cap having Vthe 'surface which 'is away from said jet nozzle coated with a material of higher emissivity than the material of said cap and the surface adjacent the jet nozzle reflective thereby to reect some of the heat emanating from the jet nozzle which would thermally affect said cap, and at least two reflective shields mounted in spaced relation to each other and between said cap and said skirt for reducing the transfer of heat to said cap.

No references cited. 

1. IN A VAPOR VACUUM PUMP, A PUMP CASING HAVING AN INLET, AN OUTLET, AND A BOILER FOR HEATING PUMP FLUID, A NOZZLE ASSEMBLY MOUNTED IN THE PUMP CASING FOR PROVIDING A VAPOR JET STREAM, SAID NOZZLE ASSEMBLY HAVING AT LEAST ONE JET NOZZLE COMPRISING A LIP PORTION AND A SKIRT BETWEEN WHICH THE JET OF VAPOR IS EXPELLED FOR PUMPING ACTION, A CAP SURROUNDING SAID JET NOZZLE IN SPACED RELATION TO SAID SKIRT AND BEING FORMED WITH A GENERALLY VERTICAL DEPENDING EDGE PORTION EXTENDING BELOW SAID SKIRT AND INTO THE FRINGE PORTIONS OF THE JET STREAM, SAID CAP HAVING THE SURFACE WHICH IS ADJACENT THE JET NOZZLE REFLECTIVE THEREBY TO REFLECT SOME OF THE HEAT EMANATING FROM THE JET NOZZLE WHICH WOULD THERMALLY AFFECT SAID CAP, AND AT LEAST TWO REFLECTIVE SHIELDS MOUNTED IN SPACED RELATION TO EACH OTHER AND BETWEEN SAID CAP AND SAID SKIRT FOR REDUCING THE TRANSFER OF HEAT TO SAID CAP. 